"You are brought before a race of hyperintelligent extraterrestrials and charged with the task of presenting a single argument of why they do not have the right to eat us. What do you say?"
(1) If pain is bad, important, real, and irreducible, then pain has intrinsic dis value
(2) Pain is important, real, and irreducible
(3)If pain has intrinsic dis value, then any rational being who understands the importance of pain should also understand such dis valus is a reason not to cause unnecessary pain in others
(4) the Aliens are hyper intelligent, rational beings
(5) Follows that these Aliens should be well aware (probably more so than we are) that eating us is going to cause significant pain.
As far as (2) goes, I think it is safe to say we all know that pain is bad. We know that the hurt pain causes is bad for us, and as such it is also bad for others. People don't describe pain as "something great". If they do, they don't know the proper meaning of the word "pain". We also know that pain is important, that it is something to be avoided in and of itself. We don't treat someone with a severe acid burn for other reasons like cosmetic appeal-we do it because the pain itself deserves alleviation. It is an end in itself. In addittion, the disvalue of pain is no different than any other non reducible mental state. Pain feels bad and that's that. In other words, we don't describe the badness of pain in non moral terms: to know why pain is bad you have to experience it. Lastly, pain is real because we experience it.
I'm sure some of you still might assume that if the Aliens could wipe us out instantly, causing no pain, then what they do with us shouldn't be an issue. Fair enough. That would be quite the practical feat. Other arguments concerning the nature of justice would have to come into play.